The present invention relates to an image forming apparatus.
In an image forming apparatus in which a toner image is formed on an image carrier such as photoconductor or transfer belt and then transferred onto a recording sheet by electrostatic force of a transfer roller or other transfer member, a top layer of the transfer member is formed of resin foam having both elasticity for slight deformation to ensure a nip (pressure contact) against the image carrier and electrical conductivity for enough chargeability to attract the toner. Such a resin contains low molecular weight materials kept out of chemical reaction in manufacture processes as well as such additives as vulcanizers, softeners or plasticizers. Keeping the transfer member in a prolonged stop as it is nipped against the image carrier causes a bleed phenomenon that those low molecular weight materials and additives would bleed out in an oil-like state from the transfer member. The bled low molecular weight materials and additives would be deposited on the image carrier to worsen its surface characteristics, resulting in occurrence of noise in the formed image. Due to this, conventional image forming apparatuses are equipped with a contact-and-separation mechanism for keeping the transfer member separated from the image carrier during stops.
Some monochrome image forming apparatuses use a transfer member which is formed of an electronically conductive resin having carbon dispersed therein for less occurrence of the bleed, with the contact-and-separation mechanism for the transfer member omitted. However, electronically conductive resins have a defect of being nonuniform in electroconductive characteristics. For color image forming apparatuses, which are required to have expressive power for correct tonal gradation, it is desirable to use an ion conductive resin which is superior in electroconductive characteristics, and omitting the contact-and-separation mechanism for the transfer member would inevitably lead to occurrence of the bleed phenomenon.
With a view to avoiding any effects of occurrence of such a phenomenon due to the developing roller, JP2001-34115A describes an invention in which the image carrier is turned to remove deposits before the start of image formation. JP2000-321932A describes an invention in which the image carrier is turned little by little even during a stop to thereby reduce nonuniformities in surface characteristics. JP2003-98934A describes an invention in which with an image carrier and a cleaning member kept in a constant positional relation during a stop, portion of the image carrier with which the cleaning member comes into contact during a stop of the image carrier is excluded out of use in the image formation in order to avoid effects of such phenomena as described above due to the cleaning member. Further, JPH5-27615A describes an invention in which cleaning of the transfer member is stopped so as to make toner interventionally provided in the nip so that bled low molecular weight materials, additives and the like are adsorbed to the toner deposited on the transfer member, thereby reducing the effects of the bleed.
In the invention of JP2003-98934A, since part of the image carrier cannot be utilized at all for the image formation, intervals between image formations would be prolonged, giving rise to a problem of lowered image formation speed.